The invention relates to an optical information carrier comprising an optically transparent layer and a reflective layer adjacent to each other, and the interface between the two layers comprises an optically readable information structure containing alternate areas with such different properties, that a scanning light beam is modulated in correspondence with the information present in said structure.
Information carriers of this type in a broad sense of the meaning are known for various purposes and in various embodiments. Examples of such information carriers are for instance optical lattice grids, holograms and audio discs or video discs (compact disc).
A general problem with these known optical information carrier structures is the fact that in general gases and vapours, especially water vapour, are able to diffuse into the optical transparent layer but not through the reflective layer. Depending upon the environmental conditions, especially during a reduction of the atmospheric pressure and/or the water vapour concentration, water vapour and/or other gases will diffuse to the outside. At that side of the optical transparent layer directed to the atmosphere there are no obstructions for such a diffusion. However, at the side where the reflective layer is realized, said diffusion causes problems. The reflective layer in most cases consists of metal which does not transmit gases and vapours. In case the vapour pressure at the interface between the optical transparent layer and the reflective layer increases under the influence of environmental conditions, then it is not inconceivable that the adherence between both layers is locally not able to withstand this increase in pressure and will collapse. The result thereof is the development of larger or smaller bubbles or blisters at the interface between both layers. Such blisters form a serious hindrance for the error-free reading of the information present in the carrier structure. Taking into account the sometimes very small dimensions of the information structure, the dimensions on a video disc are in the order of some .mu.m, the development of even the smallest gas or vapour bubbles in said interface must be prevented.
The presence of a diffusion-tight reflective layer can also result in a distortion of the information carrier as a whole, because the vapour profile of a layer will in general be symmetrical in the direction towards the side edges, assuming that the same atmospheric pressure prevails at both sides and that a free exit and respectively a free penetration of gas and vapour is possible. However, if one side of the layer is closed in a vapourtight and gastight way, such as with the optical information carrier of the above-mentioned type, then in general an asymmetrical dampness or vapour profile will be developed in the optical information carrier. Such asymmetrical vapour profile causes mechanical tensions in the carrier, which may result in a mechanical distortion of the carrier as a whole, which phenomenon could be indicated by the term "warping". With an optical information carrier, as for instance a compact disc, this warping phenomenon will soon lead to a situation in which the information read-out apparatus is no longer able to read the stored information.